Journal of Electron Spectroscopy and Related Phenomena, 54155 (1990) 1115-1122 Elsevier Science Publishers B.V., Amsterdam
Energy-Dependent J. E. Rowe,
Vibrational
R. A. Malic,
Spectra
E. E. Chaban,
of the Si(lll)-B
R. L. Head&k
1115
Surface
and L. C. Feldman
AT&T Bell Laboratories Murray Hill, New Jersey 07974 Abstract High resolution electron energy loss spectroscsopy measurements have been performed on These samples have a flX& periodicity with heavily boron doped Si(ll1) surfaces. boron in the second complete layer. Three vibration losses are observed at 59, 107, and 168 meV and are due to dipole allowed modes of Si-Si, B-Si and a combination mode. The combination mode is quite strong and indicates a large anharmonic coupling between the B-Si and Si-Si modes. At higher incident electron energies, Eo215 eV, a broad surface plasmon mode is observed at -180 meV loss energy due to the free carriers in the region below the B-reconstructed surface layer.
1. INTRODUCTION. Surface vibrational spectra of adsorbates on metals have been probed extensively by the High Resolution Electron Energy Loss Spectroscopy @REELS) method [l-3] but there is comparably less known about the vibrational spectra of adsorbates on semiconductor surfaces. An important exception to this statement the pioneering work on H adsorbed on Si and Ge surfaces using IR techniques HREELS studies Chabal and co-workers. [4-61 Th ere have also been numerous
is by
of as well as clean surfaces of III-V compound the clean Si(111)7x7 surface semiconductor surfaces [7-lo] Some of these studies have reported surface plasmon loss spectra as well as surface phonon loss spectra. In this paper, we report HREELS data for the system B-Si(lll) in a dipole scattering geometry. The energy dependence of the dipole excitations is due to the energy dependence of the spatial penetration of the incident electron wave field. We use this energy dependence to show that surface plasmons can be excited below tht strongly perturbed This surface region is -3.5A thick and is highly reconstructed surface region. conducting; thus the low energy wave field is almost completely screened. Three phonon modes are observed for this surface at 59, 107, and 168 meV as well as weak evidence for a low energy mode at -30 meV. 2. SAMPLE
PREPARATION
AND
CHARACTERIZATION.
High resolution electron energy loss spectroscopy with specular scattering geometry has been used to investigate the l/3 monolayer Si(lll>B flXfl This surface can be prepared by B segregation of heavily doped surface. (-1.5X 1020 cm- 3, Si(ll1) wafers. [ll] A number of recent structural
036%2048/90/$03.50
0 1990 Elsevier Science Publishers B.V.
1116
measurements have confirmed that the l/3 monolayer B-Si(111) flxfl R30” surface has the B atoms in a second layer subsitutional site with l/3 monolayer of Si adatoms directly above as a fifth neighbor in the first near-neighbor shell. (11-141 0 ur samples were Si(lll) wafers implanted with 30 keV B” to 2x10” cmB2 followed by annealing at 1050’ C for 90 min. The wafers were then cleaned chemically and the surface prepared by chemical growth of a thin protective oxide layer before transferring into the ultrahighvacuum HREELS system. The TOP VIEW boron (fix mR30 ’ surface reconstruction was prepared in UHV by heating the Si sample to 900 a C which desorbed the oxide layer and allowed diffusion of l/3 monolayer B SIDE VIEW [d to the surface. The resulting surface was extremely well ordered and I__ stable showing no [Go] meaureable changes of contamination for several Fig. 1. Structural model for the Boron &Xfi hours. surface showing both top view (upper panel) and side view (lower panel). The boron atoms are The thin reconstructed surface indicated by solid circles and the silicon atoms are layer was studied by the authors indicated by open circles. The Si adatoms are using photoemission spectroscopy located directly above the boron subsurface sites and with synchrotron radiation P51 are indicated by larger open circles. which confirmed other recent studies showing that the B-reconstructed region is about 3.5-4.5A thick or about three atomic layers. Our measurements show a metallic Fermi edge (see Fig. 2) that indicates that this surface region is highly conducting and has a Fermi-level position at the bulk valence band maximum. Thus there is no space-charge depletion as there is on Si(lll)7x7 surfaces and one should oObserve free carrier plasmon excitations close to the surface and extending lOO-1OOOA into the bulk. A schematic diagram of the spatial dependence of the B and hence, of the carrier concentration is shown in Fig. 3. Note that the distance scale for the reconstructed surface region has been exagerated for clarity (it is multiplied by a factor of 20 in the scale of Fig. 3). The density in the surface region (if it correspond: to one carrier per boron) would have a Thomas-Fermi screeening length of Xsx1.5A.
1117
Thus the 3 monolayer thickness of this layer [10] is sufficient to fully screen the incident electron wave field while the electron is outside the surface. The monolayer I/3 concentration of boron was verified experimently using secondary ion mass spectrometry (SIMS), [17] but the effective thickness could not, be accurately determined by SIMS due to its inherent depth resolution. 3. VIBRATIONAL OF B-Si(ll1).
SPECTRA
Typical high resolution electron energy loss spectra with specular scattering geometry for the Si(lll)B flXJ3 surface for an incident angle of 60 ’ and sample temperature of 110 K are shown if Fig. 4. The data were collected with a digital ramping PC based data acquisition system at a point, density of ~0.5 meV and then smoothed with a standard 7-point algorithm. Three well defined loss peaks are found at energies of 59, 107, and 168 meV. The 59 meV mode is identified as a Si-Si mode and is close in energy to Si-Si modes on Si(lll)Sxl surfaces which are 56 meV [18] and To similar modes on Si(111)7x7 (63 meV) (191 and Si(lll)lxl (69 meV) [20]. The 107 meV mode is identified as the B-Si stretchin (or breathing) mode and is strongest in this qeometry because of the charge difference between B and Si. This mode is close to the energy reported for C-Si phonons (112 meV) [7] but is narrower and more symmetric. This is presumably because the B is in a single site at this surface while the C is found to
Fig. 2. Ultraviolet photoemission spectra for a normal emission collection geometry showing the weak Fermi edge which indicates that the boron surface region is “metallic” and thus screens the field of the incident electron used in CREELS. ’
a-Si(ll1)
~00 do0 -100 do0 do0 a iloo -am -100 o DlatalNX EWWti sullam (A) Fig. 3. Schematic diagram of the boron distribution m our samples showing a l/3 monolayer region of boron at the surface and a heavily doped -1.5% boron cvncentration below the surface region which
is N 3.5A thick.
1118
be in a distribution of sites below the surface, perhaps extending to - 10’s of angstroms. [7] The higher energy mode at 168 meV occurs at nearly the sum of the energies of the other two modes (166 meV) and therefore is assigned to a combination mode of B-Si and Si-Si phonons. This is allowed because of the anharmonic coupling of these two modes and should be strongly temperature dependent if the coupling is weak. The intensity is same for essentially the temperatures in the range 110-300 K and thus the coupling is not first order. The strength of this mode is stronger than the Si-Si mode which may be due to the coupling to Si-Si modes that are polarized more nearly parallel to the surface than perpendicular. There is also the strong probablity that all Si atoms except, *for the adatom are charged nearly the same [16] as determined by Si core-level photoemission and all Si-Si modes have therefore weaker dynamic dipole cross sections than those of other Si surfaces. Shown as a dashed line in Fig. 4 at E=O is the elastic peak in the straight-through mode which is 3.5 meV in width. The reflected elastic peak width is much wider -12 meV and the B-Si phonon is -20 meV wide. Since our samples are heavily doped we suggest, that low energy are pair excitation eletron-hole strongly coupled to all the phonon modes as well as the elastically The Fermi electrons. scattered energy
for
the
near
surface
Fig. 4.
High Resolution Electron Energy Loss Spectroscopy (HREELS) at 1.0 eV incident energy showing the three vibrational modes that we observe for this surface at 59,107 and 168 meV.
Fig. 5. Schematic energy band diagram for bulk carriers in Si showing the indirect bandgap E&=1.2 eV at -100 K and the Fermi energy for holes corresponding to our heavily B-doped samples. The spin-orbit splitting is 35 meV at k=O.
bulk
splitting of the energy for the near surface bulk is -13 meV and the spin-orbit valence band is 35 meV. A schematic bulk band structure diagram for Si at this
1119
carrier concentration is shown in Fig. 5. As can be seen a large number of intravalence band excitations are possible. This additional broadening is analogous to that previously found for low energy space-charge layer plasmons. (21-231 4. DISCUSSION
AND
CONCLUSIONS.
In addition to the surface phonon modes at 59, 107, and 168 meV it is of some interest to determine the two electronic plasmon modes of this system which we identify from the loss energy and relative cross sections. The energy dependence of the specular scattering results are shown below in Fig. 6. For higher energies the phonon loss peaks are suppressed in accordance with the expected dipole cross section which decreases as E-%, [l] and a new loss peak appears at 180 meV for the Ec=30 eV incident electron energy data. \. This is assigned to the freecarrier surface plasmon. The surface plasmon energy is consistent with a bulk doping concentration of -7 x 101gcm-3 using the bulk-Si low energy dielectric constant of 11.7 and a bulk hole mass of 0.81m,. We identify this as due to the boron the below in region the reconstructed surface (see Fig. 3) and we assume that at these high doping levels only about OS0 300 sso -so 0 so 100 one-half of the boron impurities Loss Energy(mev) are electrically active and thus Fig. 6. Vibrational spectra for three different rise give to a free-carrier incident energies 1,3, and 30 eV showing the plasmon mode. The other mode decrease in intensity of the dipole geometry loss to be a two is expected peaks from the BSi(ll1) surface and the increase in dimensional plasmon similar to intensity of the free carrier surface plasmon peak at that seen for thin metallic 180 meV with increasing electron energy. overlayers [24] or for strong accumulation layers. [25] This 2-D plasmon has a qualitatively different dispersion relation from the surface plasmon and should appear in our specular scattering data as a broadening. [25] Thus at present we have not identified this mode; however, the broad background signal present in all spectra is consistent with this mode.
n
1120
The B-Si surface optic phonon energy is considerably higher than the bulk B-Si local mode26 energy of 79 meV but agrees with a “mass-defect” model which gives 102 meV using the Si-Si measured optic phonon energy of 56 meV from Si(lll)2xl and the (M=lO) mass of the Boron isotope in our samples. For example, the adatom vibrational mode energy can be compared to that of the Si(111)7x7 surface in order to determine the influence of the second neighbor B atoms on this mode. There is weak evidence of an Si-Si adatom phonon mode near the expected value of -30 meV obtained from scattering on the Si(111)7x7 surface. [20] At low incident electron energy the plasmon modes are suppressed since they are polarized longitudinally in the surface plane and we observe another phonon mode at 168 meV which is assigned to a combination mode of the B-Si phonon and a Si-Si phonon at -59 meV. The small energy shift from the expected value of the sum of the individual mode energies may be due to anharmonic effects. The energy widths of these phonon modes are quite broad in the present samples even after several annealing cycles and this may be due to additional coupling of these modes to electron-hole pair excitations[4] rather than due to static disorder. Additional measurements are in progress on less heavily doped samples and using lower temperatures in order to reduce the broadening effects. ACKNOWLEDGEMENTS We would like to acknowledge useful discussions with many of our collegues: D. R. Hamann, Y. J. Chabal, Y. J. Ma and S. B. Christman of AT&T Bell Laboratories and L. L. Kesmodel of Indiana University.
1121
REFERENCES 1. H. Ibach and D. L. Mills, Electron Energy Loee Spectroscopy Vibrations (Academic Press, New York, 1884).
oj Surface
2.
J. T. Yates, Jr. and T. E. Madey, eds., Vibrational on Surfacea (Plenum Press, New York, 1987).
3.
A. M. Bradshaw and H. Conrad, eds., Vibrations Press, New York, 1987).
4.
Y. J. Chabal, Phys. Rev. Lett. 50 (1983) 1850.
5.
Y. J. Chabal and K. Ragavachari, Phys. Rev. Lett. 54 (1985) 1055.
0.
Y. J. Chabal, Surf. Sci. Repts. 8 (1988) 211.
7.
H. Froitzheim, U. Kohler, and H. Lammering, Phys. Rev. B 30 (1984) 5771.
8.
B. N. J. Persson and J. E. Demuth, Phys. Rev. B 30 (1984) 5968.
9.
J. M. Seo, D. S. Black, P. H. Holloway, and J. E. Rowe, J. Vat. A 6 (1988) 1523.
Spectroscopy
of Molecules
at Surfaces 1987 (Elsevier
Sci. Technol.
10.
H. Luth, Physics 117B & 118B (1983) 810.
11.
R. L. Headrick, I. K. Robinson, E. Vlieg and L. C. Feldman, Phys. Rev. Lett. 63 (1989) 1253.
12.
P. Bedrossian, R. D. Meade, K. Mortensen, D. M. Chen, J. A. and D. Vanderbilt, Phys. Rev. Lett. 63 (1989) 1257.
13.
I. W. Lyo, Kaxiras and Ph. Avouris, Phys. Rev. Lett. 63 (1989) 1261.
Golovchenko
1122
14.
H. Huang, S. Y. Tong, J. Quinn and F. Jona, Phys. Rev. B 41 (1990) 3276.
15.
J. E. Rowe, R. L. Headrick and L. C. Feldman (to be published). Kaxiras, K. C. Pandey, F. J. Himpsel, and R. M. Tromp, Phys. (1990) 1262.
16.
A. B. McLean, 7604.
17.
R. L. Headrick, A. F. J. Levi, H. S. Luftman, Feldman, Phys. Rev. B 42 (1990) in press.
18.
H. Ibach, K. Horn, R. Dorn, and H. Luth, Surf. Sci. 38 (1973) 433.
19.
J. Demuth, B. N. J. Persson, (1983) 2214.
20.
W. Daum and H. Ibach, Phys. Rev. Lett. 59 (1987) 1593.
21.
J. A. Stroscio
22.
B. N. J. Persson
23.
L. H. Dubois, 9128.
B. R. Zegarski,
and B. N. J. Persson,
24.
L. H. Dubois, (1984) 3208.
G. P. Schwartz,
R. E. Camley and D. L. Mills, Phys. Rev. B 29
25.
H. Yu and J. C. Hermanson,
26.
M. Chandrasekhar, H. R. Chandrasekhar, Phys Rev. B 22 (1980) 4825.
L. J. Termineilo,
and F. J. Himpsel,
Phys.
Rev. B 41 (1990)
J. Kovalchick,
and A. J. Schell-Sorokin,
also see E. Rev. B 41
Phys. Rev.
and L. C.
Lett. 51
and W. Ho, Phys. Rev. Lett. 54 (1985) 1573. and J. E. Demuth,
Phys. Rev. B 30 (1984) 5968. Phys. Rev. B 35 (1987)
Phys. Rev. B 41 (1990) 5991. M. Grimsditch
and M. Cardona,